Ten to 17% of children 4-16 years have recurrent abdominal pain (RAP). Of these, up to 68% meet the criteria for irritable bowel syndrome (IBS), which also affects 15- 20% of adults. Up to 66% of children with RAP develop IBS as adults. This project focuses on developing treatments and understanding the pathophysiology in children with RAP (IBS-type). Using a randomized, double blind, placebo controlled design, our Specific Aims are, in children with RAP-IBS, to: 1) Characterize the effectiveness of soluble fiber versus a probiotic compared with placebo in improving symptoms (number of pain episodes, mean rating of pain severity, pain-induced interference with activities, and percent of stools that are normal; primary endpoints); 2) Identify potential mechanisms whereby soluble fiber or a probiotic improve symptoms in children with RAP-IBS by measuring gastrointestinal (Gl) function tests: Gl transit time, breath hydrogen production, Gl permeability, and fecal calprotectin concentration; secondary endpoints); and 3) Define the potential contribution of psychological constructs (child anxiety, somatization, coping, and parental somatization, coping, and illness interaction) in explaining the response to the treatments (number of pain episodes, mean rating of pain severity, pain-induced interference with activities). We Hypothesize that: 1) Administration of soluble fiber or a probiotic will improve reported symptoms associated with RAP-IBS compared with placebo; 2) Soluble fiber and/or a probiotic will improve Gl function tests: normalize Gl transit time, decrease breath hydrogen production, Gl permeability, and fecal calprotectin concentration; and 3) After controlling for the effects of treatment and Gl function tests, psychological constructs will serve as moderators of treatment effects as measured by a change in the number of pain episodes, mean rating of pain severity, and interference with activities. The results of this proposal likely will benefit the large numbers of children with RAP-IBS and potentially provide insight into its prevention or symptom amelioration. Identification of factors influencing symptoms in children is likely to apply to adults. Interventions in childhood may prevent or ameliorate symptoms in adults by changing the physiological and the learning/conditioning and adaptive experience of these children. We anticipate these data can used to address the significant emotional and economic burdens of these disorders, which in adults is near eight billion dollars per year.